ISIS have received heavy bombardment in Palmyra for the second day in a row, a group monitoring the war has said.

The Islamic State-held city of Palmyra was bombed on Friday as Syrian army jets carried out at least 25 air strikes. The Syrian Observatory for Human Rights said it was one of the most sustained government bombardments of Palmyra.

The air strikes killed at least 26 people, including 12 Islamic State fighters, the British-based Observatory said.

On Thursday, Syrian jets had carried out at least 12 air strikes on Raqqa, Islamic State's de facto capital in the north. Elsewhere airstrikes killed 17 people in Idlib, the Observatory said. 

The city of Idlib is the base of an alliance of jihadist and Islamist groups calling itself the Army of Conquest.

Syrian forces had started using new types of "very accurate" air and ground weapons supplied by its ally Russia, a source told Reuters. 

The possibility of greater military involvement by Russia has alarmed the United States, which is leading a coalition that has been bombing Islamic State strongholds in both Syria and Iraq.

The US government rejects the idea, advocated by Moscow, of cooperating with Syrian President Bashar al-Assad to fight Islamic State, and has warned Syria not to interfere in its air campaign.

However, the Russian and US defence ministers spoke by phone on Friday for the first time in over a year. The Pentagon said such talks were necessary to avoid "miscalculation".

U.S. Secretary of State John Kerry said President Barack Obama saw military talks with Russia on Syria as an important step. 

"Our focus remains on destroying ISIL (Islamic State militants) and also on a political settlement with respect to Syria, which we believe cannot be achieved with the long-term presence of Assad," he told reporters. "We're looking for ways in which to find a common ground."

Additional reporting by Reuters.